Log an image as an error if imbo didn't return an image identifier.

In certain cases the path ImboClient tries to request might return
200 OK, even if the image wasn't added on the receiving end. This
patch checks if we have received an actual image identifier before
assuming that everything went OK.
Only ask for confirmation if more than five images are being added.

Having to answer "Y" to add a single image or a small collection of
images hinders an effective workflow when testing or doing small
changes.
Only ask for confirmation before uploading after scanning a directory structure.
Merge pull request #7 from matslindh/fix-addimage-issues

Fix addimage issues